1. Field of the Invention
This invention relates to electroacoustic apparatus, particularly to bulk mode acoustic delay devices.
2. Description of the Prior Art
In the prior art, multiple reflections from bulk acoustic waves in delay lines have caused undesirable output signals. A bulk mode acoustic delay line having a transducer at either end of a substrate provides a desired output signal after a bulk acoustic wave is generated and travels from the input transducer to the output and an undesired triple transit output after the bulk acoustic wave is reflected at the output and travels back and forth through the delay line to the output again. The triple transit output causes an interference ripple in the band pass of the delay line corresponding to the difference in the delay between the direct output and the triple transit output. The interference period for a triple transit output is .DELTA.f=1/2.tau. where .tau. is a delay time of the delay line. One example of a bulk acoustic delay device for delaying signals and for providing triple transit suppression is described in U.S. Pat. No. 4,099,149 issued on July 4, 1978 entitled "Bulk Acoustic Delay Device" by Bruce R. McAvoy and assigned to the assignee herein. In U.S. Pat. No. 4,099,147 the input transducer provides a predetermined defraction in the bulk acoustic wave launched causing the bulk acoustic wave to spread laterally as it propagates through the medium. The bulk acoustic wave impinges upon a receiving transducer to provide a direct output. The bulk acoustic wave after traversing the delay line three times impinges upon the receiving transducer and an additional transducer spaced outwards to intercept only the triple transit wave. The triple transit output is attenuated due to a difference in the acoustical path lengths from the input to the two receiving transducers to provide a phase difference of the two received singles. Bulk acoustic waves traveling through the medium also exhibit path attenuation due to the distance the wave travels which also reduces the triple transit signal.
An alternate approach to provide triple transit suppression in a bulk acoustic delay line is described in a paper entitled "Wideband Microwave Acoustic Delay Line With Exceptionally Smooth Phase and Loss Response" by W. R. Sperry, E. P. Kirchner and T. M. Reeder published in the digest of the International Microwave Theory and Technique Symposium sponsored by the IEEE on May 17, 1971. In the paper, triple transit suppression is enhanced by using an angled end face to place the null in the radiation pattern of the triple transit signal, launched at the input transducer aperture, at the position of the output transducer aperture. In other words, the beam axis of the bulk acoustic wave launched by the transducer is tilted or directed such that on the third transit the main beam energy misses the receiving transducer and only the side lobes are seen. The beam axis tilting approach is useful only where the input and output transducers are in the far zone relative to each other so that the acoustic beam is in the Fraunhofer field region at the output transducer.
The use of additional receiving transducers in a surface acoustic wave delay device to provide triple transit suppression by means of phase interference has been described in a paper entitled "Triple-Transit Suppression in Surface-Acoustic-Wave Devices" by M. F. Lewis published in Electronic Letters, Volume 8, Number 23, pages 553-554 on Nov. 16, 1972. In FIG. 2, two dummy interdigital transducers are spaced an additional distance of .+-..gamma./4 to reflect back to the input transducer two signals which are .+-.180.degree. out of phase with the reflection of the wave from the output transducer. The reflected waves are cancelled at the input transducer leaving no surface acoustic wave to be reflected by the input transducer to become the triple transit at the output transducer. Hulling the reflected signal at the input transducer will not provide triple transit suppression in bulk mode acoustic delay lines.
It is therefore desirable to provide electroacoustic apparatus for delaying signals and for providing wide band triple transit suppression in bulk acoustic delay lines about a predetermined center frequency.
It is further desirable to provide bulk acoustic wave delay lines that can delaly signals of an extremely broad band width such as an octave bandwidth with triple transit suppression for delays of 0.5 microseconds or less.
It is further desirable to combine a number of bulk acoustic delay lines having different acoustic path lengths to act as one delay line having triple transit suppression.